Underground retorting for secondary oil recovery



SePt- 20, 1955 w. o. HEILMAN' ETAL UNDERGROUND RETORTING ROR SECONDARY O11. RECOVERY Filed Feb. 6, 1952 2 Sheets-Sheet l Mmm O.

Hennes Hulman Bovernnons @www SePt- 20, 1955 w o. HEILMAN EVAL t 2,718,263

UNDERGROUND RETORTING F OR SECONDARY OIL RECOVERY United States Patent UNDERGROUND RETORTING FR SECNDARY IL RECOVERY William 0. Heilman, Short Hills, and Henry I. Qgorzaly,

Summit, N. J., assignors to Esso Research and Engineering Company, a corporation of Delaware Application February 6, 1952, Serial No. 270,218

4 Claims. (Cl. 166-11) The present invention is broadly concerned with an improved method for increasing the production of crude petroleum from oil-bearing formations containing the same. The invention is especially concerned with an underground retorting operation in which a zone of combustion is caused to advance through an oil reservoir in a controlled manner by utilizing certain wells for the injection of inert gas. By this means the oxygen content of a predetermined path is fixed and the path of the zone of combustion is controlled and extinction of the combustion process such as could result from excessive spreading or deviation from the desired path is prevented. Air and combustible gas injected with the cycled stream are capable under controlled conditions, of maintaining a combustion zone within the reservoir which combustion zone is caused to advance through the reservoir under the driving action of the cycle gas. The heat evolved by the combustion zone advancing through the reservoir is utilized to remove the oil retained in the porous formation and this oil is caused to flow toward the point of recovery by the action of the cycle gas.

ln conventional crude oil producing operations, situations are frequently encountered wherein oil will not produce at practical rates, even though it is known with certainty that all available, recoverable oil has not been obtained from the field. It has long been recognized that the recovery of crude oil from underground reservoirs is imperfect in that substantial quantities of the original oil, amounting to between about 2O and 50% or more, are unrecovered by the usual techniques of primary and even secondary recovery methods, such as by gas injection and by water flooding. This incompleteness in oil recovery is the result of a number of factors.

Generally speaking, in fields not subject to an active water drive, the pressure remaining in the oil-producing strata may not be sufficient to cause oil to flow toward and into the well bore hole at a practical and economic rate. Thus, the wells of an oil field tend to become exhausted with respect to primary recovery methods, even when itis known that relatively large quantities of oil are still retained in the reservoirs.

In oil fields characterized by a significant degree of structural relief, it is common practice to extend the active life of the field by maintaining its pressure arti.

ficially, as by injecting gas into the gas cap of the reservoir or by injecting water into the porous formation at a level below that of the producing wells. In addition, it is commonly known to increase the life of a field beyond the normal limit by secondary recovery methods such as by water flooding, in which operation water is continuously injected into the producing sand through injection wells. The injected water displaces a portion of the residual oil and moves the oil forward into production wells. A similar type of operation may be carried out with gas recycled at low pressure. In both of these latter procedures, the motion of the oil forward to producing wells after an initial displacement results from the frictional drive of the injected fluids on the residual the present invention, the

2,718,263 Patented sept. 2o, 1955 ice oil. As a result of these relatively ineffective methods of removing oil from the porous sand, substantial quantities of oil remain behind, even after such secondary recovery operations have in turn reached their economic limits. These in general reflect the attainment of a flowing ratio of injected fluid to recovered oil at the producing well which is so high that the expenses of the operation are not balanced by the income derived.

The gas and water drives heretofore discussed have generally employed. relatively low pressures.

More recent proposals for secondary recovery of oill from reservoirs have included the suggestion that gas be recycled at extremely high pressures. Under thesev conditions, improved recovery of residual oil is obtained because of the increased volume and decreased viscosity of the oil in the reservoir due to solution of components of the gas in the reservoir oil, and due to the increased tendency for vaporization of oil in the high pressure gas.- The latter phenomenon is an attribute of operation in the retrograde condensation range, in the neighborhood of the critical phase conditions of the reservoir hydrocarbon mixture. While the high pressure method is partially effective in obtaining increased oil recovery, it requires large capital investment and complete recovery is not attained.

Another procedure is to fill the entire oil reservoir, preferably at increased pressure, with a light hydrocarbon or hydrocarbon mixture or a relatively low viscosity sub stance soluble in the oil and then to flush out the reservoir with water. In general, the injected solvent is caused to dissolve in the oil which lessens its viscosity and increases its volume, and thus increases its ability to flow through the formation under the driving action of the water flood. As an alternate to using a single hydrocarbon such as propane alone, the use of natural gas containing light hydrocarbons such as ethane, propane or butane and the use of non-hydrocarbon solutes such as CO2 has also been proposed for this purpose.

It has also been suggested that thermal means be employed to increase the recovery of oil from underground reservoirs. However, the indiscriminate use of heat for such a purpose possesses many inherent disadvantages, primarily in the high thermal requirements required for bringing the entire oil-containing formation to the desired temperature and of supplying the heat lost to the surrounding rock formations. However, in accordance lwith heated zone is confined to a relatively small element of the formation with a proportionally small heat load, and is caused to advance through the oil-containing formation in such a way. as to achieve throughout the formation the beneficial results of elevated temperature in releasing the oil from the porous rock. The present process involves the generation of heat by means of combustion supported by the interaction or' oxygen-containing gaswith a fuel in order to produce the elevated temperature within the zone. This combustion zone is caused to advance through the formation by the convection of heat in the direction of gas flow and serves to ensure a complete clean-up of the oil contained in the formation by effecting vaporization of the residual oil. This oil is carried forward as a vapor by the combustion gas, and condensed in the cooler portions of rock ahead of the zone of elevated temperature; the liquid oil is thus forced into the producing wells by the driving action of the gas.

The process of the present invention may be more readily understood by reference to the drawings illustrating the embodiments of the same. i

Y Figure l is a profile of a producing formation iin a disposition of the wells or'bore holes for producing the field.

Referring specifically to Figure l, a gas having an oxygen concentration substantially less than undiluted air is introduced into the oil producing formation by means of an injection well 1. The oxygen concentration is in the range from about 3% to 15%, preferably in the range from about 5% to 10%. This concentration of oxygen may be secured by diluting air with an inert gas such as nitrogen or with combustion gas. The oil producing formation is disposed between an upper cap rock 2 and a lower cap rock formation 3. The oxygen containing gas may also contain a suitable proportion of combustible gas such as natural gas or other fuel capable of being vaporized in the injected gas stream. In accordance with the concept of the present invention a combustion zone 4 is caused yto pass through the formation from the injection well 1 to the producing well 5.

The temperature of the porous rock rises from the point of the injection well to the combustion zone 4 and falls from the zone of combustion 4 to the producing well 5. Thus, as the injected gas flows through the formation from the injection well to the zone of combustion, it is heated by contact with rock as it approaches the burning Zone. When the temperature of the gas stream fiowing through the porous formation reaches ignition temperature levels, combustion of the fuel with the oxygen of the gas stream occurs with the release of heat. As the hot gas stream passes through the combustion zone into the cooler rock formations in the direction of the producing well, the gases are cooled by direct contact with the rock formation; and this transfer of heat from the rearmost portion to the foremost portion of the combustion zone results in a forward advance of the zone of high temperatures.

As the hot zone moves forward through the porous formation, residual oil is vaporized with the heaviest fractions undergoing a form of retorting. These vapors are carried forward by the gas stream into the cooler portions of the formation ahead of the combustion zone, where they are condensed and driven forward by the action of the liowing gas. The mixture of gas and oil enters the producing well and reaches the surface as a gas stream containing entrained droplets of oil. This mixture is passed through a gas separator from which the produced oil is Withdrawn by means of line 21 and from which the gas passes by means of line 22 to a booster compressor 23 for recycling to the injection well 1. A portion of gas, corresponding in quantity to the air and natural gas which are added to supply the oxygen and fuel required to maintain the combustion, is vented from the separator by means of line 24. The desired quantity of natural gas may be added by means of line 2S while corresponding quantities of air may be introduced by means of line 26.

It has been found that a temperature in the combustion zone above about 800 F. is required to effect the combustion of natural gas in the underground formation. A temperature of approximately 1000 F. is adequate to obtain substantially complete combustion of natural gas. With hydrocarbon fuels of higher molecular weight, lower temperatures may be satisfactorily employed, for example, at temperatures of 500 F. to 600 F. residual oil fractions will burn at a high rate. Consequently, the process may be operated without the addition of natural gas or other extraneous fuel to the injected gas stream by using a portion of the oil contained in the reservoir to fuel the combustion process. Where carbon is deposited in the porous formation by retorting of the residual oil in place, a portion of the heat requirement may also be supplied by combustion of the solid carbonaceous matter.

The heat released by the combustion of fuel within the hot zone is generated within an element of relatively small dimension in the direction of flow, which is maintained at a temperature sufficient to ensure a high rate of combustion. Heat released in this element iiows both forward and backward with reference to the direction of gas flow by thermal conduction through the porous rock formation and heat generated in this zone is also carried forward by the convective effect of the cycle gas stream. Equilibrium temperatures are rapidly established and there is then no tendency for the further accumulation of heat within the porous formation, that is, the heated zone does not expand. Likewise, in the process of the invention there is no loss of heat from the porous formation in the direction of gas flow because a substantially perfect heat exchange between the rock heated by the combustion and the injected gas occurs on one side of the burning zone and a similarly complete heat exchange occurs between the cool rock into which the burning Zone is advancing and the hot combustion gases. All of the heat generated by the combustion occurring in the heating zone is thus available to supply the heat which is lost from the porous formation by thermal conduction vertically into the upper and lower confining rock layers.

The quantity of heat which must be supplied in order to maintain the combustion zone temperature depends on the minimum ignition temperature of the combustible employed. lt is also affected by the thickness of the porous layer contained between the upper and lower cap rock formations. Likewise, the rate of advance of the burning zone through the formation has a significant effect on the heat losses and consequently on the amount of combustible which must be supplied. ln general, too low a rate of advance can result in a lowering of the temperature below that necessary to maintain combustion. It has been found that the rate of advance of any unit cross-sectional area of the combustion front corresponds to the rate of flow of the combustion-supporting gases in this way, that the heat capacity of the section of the formation through which the unit area of the burning front is advanced during a given time is equal to the heat capacity of the gases flowing through the unit cross-sectional area during the same period.

One problem encountered when producing a field by the method described above is that to some extent the zone of combustion tends to spread or deviate from the predetermined, desired path, which may result for the reasons given heretofore, in extinction of the burning process. 'l he method of controlling the path as desired is illustrated by Figure 2. Referring specifically to Figure 2 it is assumed that the individual wells 31 to 58 inclusive, are positioned apart anywhere from about 50 feet to 1000 yards or more. It is also assumed that due to reservoir conditions it is desired to initially retort the field in a path between injection well 48 and a producing well 49. Under these conditions wells 48 and 49 of Figure 2 will correspond to wells 1 and 5 of Figure l.

An oxygen containing gas is introduced through injection well 48 and ignited. As pointed out, the Zone of combustion may tend to advance in all directions from injection well 48. However, in accordance with the present invention, inert gases, i. e., gases containing no oxygen or an amount of oxygen insufficient to maintain combustion are introduced into the surrounding wells 43, 44, 47, 52, 55, 56, 53, 45, 50, and 57. This gas may be combustion gas or air diluted with an inert gas such as nitrogen to secure an oxygen concentration below that necessary to support combustion. Pressure conditions are adjusted so that the injected gases from well 4S will flow toward a low pressure producing well 49. Furthermore, the pressures on the surrounding wells named are also adjusted so that these gases flow in the direction of producing well 49. Thus the oxygen concentration will be sufficient to sustain combustion only in the predetermined path between wells 48 and 49.

After the area of the reservoir between wells 48 and 49 has been retorted other areas of the reservoir can be retorted in a manner similar to that described with respect to the area or path between wells 48 and 49. For example oxygen-supporting gases of the desired oxygen concentration can be introduced into injection Well 39. Inert gases are then introduced into wells 35, 31, 32, 36, 33, 34, 37, 42, 41 and 40, thus causing the zone of combustion to move in the direction of producing well 38.

It will be understood that the porous formation, while being pervious to gas, will exert a certain resistance to the ow thereof, a resistance which may be referred to as pressure drop between two given points in the formation. In order for gas to ow from one point to another it is necessary not only that a pressure difference exist between these points, but that this difference be of suicient magnitude to exceed the natural pressure drop existing at the particular conditions of temperature, gas velocity, etc., being used. Thus the pressure difference between the primary injection well 48 and the producing well 49 must be greater than the pressure drop through the formation therebetween. However, upon injecting inert gas into the secondary injection wells 43, 44, 45, 46, 47, 51, 52, 53, 54, 55, 56, 57, and 58, a pressure difference between these secondary injection wells and the primary injection well 48 is set up. In order to prevent undesired ilow of gas from the primary injection well to the secondary injection wells it is necessary that the pressure drop exceed the pressure difference. On the other hand, the pressure difference between the secondary injection wells and the producing well must be greater than the pressure drop therebetween in order to permit flow toward the producing well.

The pressures required to be exerted on the injection wells should be higher than the pressure on the producing well. The pressures required on the wells wherein inert gases are introduced should be sufficient to secure a net ow from these Wells toward the producing well. The major pressure differential in most instances will be between the well at which the oxygen-containing gas is introduced and the producing well.

In accordance with a specific concept of the present invention, combustion gases of no oxygen content secured from the producing well and segregated by means of, for example, line 24, are recycled as inert gases to the surrounding secondary injection wells 43, 44, etc.

What is claimed is:

1. Process for the recovery of hydrocarbons from porous formations containing the same and into which have been drilled a producing well, a primary injection well, and a plurality of secondary injection wells adjacent said producing and primary injection wells, comprising the steps of injecting a fuel gas into said formation by means of said primary injection well together with a gas containing an amount of oxygen suiiicient to maintain combustion within said formation, igniting the mixture of oxygen and fuel gas in said formation, imposing a first pressure difference between said primary injection well and said producing well greater than the pressure drop existing therebetween in a manner to cause the injected gases to flowin the direction of said producing well and to advance a zone of combustion in a path between said primary injection well and said producing well, injecting inert gases having an amount of oxygen insuflicient to maintain combustion within said formation into said formation through a plurality of said secondary injection wells, imposing a second pressure difference between said secondary injection wells and said producing well greater than the pressure drop existing between said secondary injection wells and said producing well to cause said latter gases to flow in the direction of said producing well, whereby flow of gases from said combustion zone toward said secondary injection wells is prevented, the pressure difference between said primary injection well and each of said secondary injection wells imposed by virtue of the imposition of said first and second pressure differences being less than the pressure drop between said primary injection well and each of said secondary injection wells, and recovering hydrocarbons from said producing well.

2. Process as defined by claim l wherein the oxygen content of the gas containing oxygen introduced into said injection well is in the range of from about 3% to 15% by volume.

3. Process for the recovery of hydrocarbon constituents from porous formations containing the same and into which have been drilled a producing well, a primary njection well, and at least one secondary injection well adjacent said producing and primary injection wells, comprising the steps of injecting an oxygen-containing gas into said formation by means of said primary injection well, igniting said formation at said primary injection well, imposing a first pressure difference between said primary injection well and said producing well greater than the pressure drop existing therebetween in a manner to cause the injected gases to ow in the direction of said producing Well, injecting inert gas into said secondary injection well to exert a pressure on said secondary injection well greater than the pressure in said producing well, thereby imposing a second pressure difference between said secondary injection well and said producing well greater than the pressure drop between said secondary injection well and said producing well, whereby the combustion zone moves in the direction of said producing well, and recovering vaporized constituents from said producing well.

4. Process as defined by claim 3 wherein said inert gases introduced into said secondary injection well comprise combustion gases recycled from said producing well.

References Cited in the tile of this patent UNITED STATES PATENTS 1,473,348 Howard Nov. 6, 1923 2,188,737 Hxon Jan. 30, 1940 2,347,778 Heath May 2, 1944 2,390,770 Barton et al Dec. 11, 1945 2,584,606 Merriam et al. Feb. 5, 1952 

1. PROCESS FOR THE RECOVERY OF HYDROCARBONS FROM POROUS FORMATIONS CONTAINING THE SAME AND INTO WHICH HAVE BEEN DRILLED A PRODUCING WELL, A PRIMARY INJECTION WELL, AND A PLURALITY OF SECONDARY INJECTION WELLS ADJACENT SAID PRODUCING AND PRIMARY INJECTION WELLS, COMPRISING THE STEPS OF INJECTING A FUEL GAS INTO SAID FORMATION BY MEANS OF SAID PRIMARY INJECTION WELL TOGETHER WITH A GAS CONTAINING AN AMOUNT OF OXYGEN SUFFICIENT TO MAINTAIN COMBUSTION WITHIN SAID FORMATION, IGNITING THE MIXTURE OF OXYGEN AND FUEL GAS IN SAID FORMATION, IMPOSING A FIRST PRESSURE DIFFERENCE BETWEEN SAID PRIMARY INJECTION WELL AND SAID PRODUCING WELL GREATER THAN THE PRESSURE DROP EXISTING THEREBETWEEN IN A MANNER TO CAUSE THE INJECTED GASES TO FLOW IN THE DIRECTION OF SAID PRODUCING WELL AND TO ADVANCE A ZONE OF COMBUSTION IN A PATH BETWEEN SAID PRIMARY INJECTION WELL AND SAID PRODUCING WELL, INJECTING INERT GASES HAVING AN AMOUNT OF OXYGEN INSUFFICIENT TO MAINTAIN COMBUSTION WITHIN SAID FORMATION INTO SAID FORMATION THROUGH A PLURALITY OF SAID SECONDARY INJECTION WELLS, IMPOSING A SECOND PRESSURE DIFFERECE BETWEEN SAID SECONDARY INJECTION WELLS AND SAID PRODUCING WELL GREATER THAN THE PRESSURE DROP EXISTING BETWEEN SAID SECONDARY INJECTION WELLS AND SAID PRODUCING WELL TO CAUSE SAID LATTER GASES TO FLOW IN THE DIRECTION OF SAID PRODUCING WELL, WHEREBY FLOW OF GASES FROM SAID COMBUSTION ZONE TOWARD SAID SECONDARY INJECTION WELLS IS PREVENTED, THE PRESSURE DIFFERENCES BETWEEN SAID PRIMARY INJECTION WELL AND EACH OF SAID SECONDARY INJECTION WELLS IMPOSED BY VIRTUE OF THE IMPOSITION OF SAID FIRST AND SECOND PRESSURE DIFFERENCES BEING LESS THAN THE PRESSURE DROP BETWEEN SAID PRIMARY INJECTION WELL AND EACH OF SAID SECONDARY INJECTION WELLS, AND RECOVERING HYDROCARBON FROM SAID PRODUCING WELL. 